Answer:
D. Learning a language can be difficult, yet it is one of the most useful hobbies that I’ve tried.
Step-by-step explanation:
The second idea contrasts with the first one. Even though learning a language is difficult, the person still had tried the hobby. Traditionally, one would not try the hobby if the hobby is difficult. However, this person has tried it. Therefore, a "but" or a "yet" should be used. A semicolon should not be used because these ideas can pair together.